Damper for hearing aids



W. A. SCHWALM DAMPER FOR HEARING AIDS Filed April 26, 1947 Nov. 7, 195o Earphone mmsuwflll IM Frequency loM Cps.

WALTER A. ScHwALM INVENTOR.

Hls Arron/VET natenteci Nov. 7, 15950 Walter A. Schwalm, Oak Park, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application April 26, 1947, Serial No. 744,196

1 claim. 1

This invention relates to hearing aid apparatus and more specically to the construction of microphones for such apparatus.

As a result of the mass and stiffness characteristics of the transducing elements included in a hearing aid microphone those elements tend to resonate at relatively high audio frequencies in the order of-4000 cycles per second. Such resonance causes the microphone to emphasize sounds in that region,.which is outside the range of frequencies required for intelligible reproduction of speech, `and satisfactory reproduction of sounds in general. Further, because of inherent structural dimension limitations, the frequency response band for the hearing aid earphone of the magnetic type is usually limited at its upper extremity to frequencies in the order of 2500 cycles. With the microphone producing maximum responseat 4000 cycles, and the earphone failing to reproduce any soundsbeyond 2500 cycles, a very unsatisfactory operating condition is liable to exist. This condition arises because the operator adjusts his hearing aid volume controlV so that he may hear satisfactorily the sounds within the band of frequencies to which his earphone responds. l Under these operating conditions sounds in the region of maximum response ofthe microphone will very likely be at such a high audio level that they will overload the final ampliiier stage in the hearing aid. This is particularly true because of the small dynamic range of the sub-miniature tubes necessarily utilized in hearing aids. Such overloading produces distortion. In addition cross-modulation will occur and spurious modulation products will appear Within the pass band of the earphone. Such spurious sounds tend to mask the desired sounds and the performance of the hearing aid is impaired. This condition of overloading in the iinal ampliiier as a result of high frequency sounds not audible through the earphone is likely to be very prevalent because such high frequency sounds are produced by the rattling of paper and other very common noise sources.

Therefore, it is an object of this invention to provide an improved hearing aid.

A further object of the present invention is to provide improved hearing aid apparatus in which the reproduction of undesired noise signals is minimized.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claim. The present invention itself, both as to its organization and manner of operation. together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with accompanying drawings in which:

Figure 1 is a diagram partially in block form of a hearing aid incorporating the present invention.

Figure 2 is a cross-sectional View taken on line 2 2 in Figure l with certain parts omitted, and

Figure 3 is a graph showing frequency response characteristics of certain elements of the device shown in Figure l.

In Figure 1, briefly, sounds impressed on microphone I are transformed to electrical impulses and those impulses are transmitted over cond-uctors 2 and 3 to amplifier 4 Where the impulses are amplified in conventional fashion and then conductedzover conductors 5 and 6 to learphone 'I Where the original sounds are reproduced.

In Figure 2, microphone I includes case8 of rigid material, such as metal, crystal 9 of Rochelle salts, or the like, possessing piezo-electric properties, support IU of electrical insulating material separating crystal 9 from case 8, diaphragm I I, preferably conically shaped, flexible in the region ofthe edges thereof .and supported about the periphery of case 8, and drive rod I2 of rigid material connected at one end to the center of diaphragm II and at the other end to crystal 9, adhesive material being used to bond the ends of rod I2 to the respective driving and driven elements. Conductor I3 is in electrical contact with one surface of crystal S and conductor I4 is` in contact With the opposite side of crystal 9. Conductors I3 and I4 connect the respective crystal surfaces to terminals I5 and I6, respectively, terminal I5 being insulated from case 8 and terminal I 6 being connected to case 8. Perforated. cap Il is of substantially rigid material and is clamped over the edges of case 8 to protect diaphragm II from direct physical pressure. Damping layer I9 is of an acoustically resistive cloth. This cloth covers cap Il and is secured thereto by means of an adhesive. A flange 20 of rubber or other resilient material may be provided around the periphery of microphone I to provide means for resiliently supporting the microphone in a hearingaid case.

As has been indicated, hearing aid microphones normally have their maximum response at a frequency outside the range of frequencies reproduced by the hearing aid earphone. In the past. this undesirable frequency characteristic has simply been accepted and tolerated.

and B indicate very clearly that sounds having 1 frequencies in the region of 4000 cycleswillcause a relatively high level of output from the microphone but they will not be reproduced by the earphone. The possibilities of. voverloading the final stage of ampliiication in the hearing aid with sounds having frequencies in this rangek is also;

clearly evident.

Curve C is drawn to the same scalel as curve x y B and shows the effect of adding the resistive cloth to the-conventional microygihone.` vkIltfvwill be noted that ,the peak at 4000 cycles hasbeen reduced yseveral, decibels, in amplitude, ,Whereas the response at the lower frequencies has not been reduced,L 5 The ultimate leiect of such a reduction jin. thev sensitivity of ther hearing 4 aid `microe phone to paper noises and other undesirable sounds inthe,regiornof` 4000 cycles is to reduce substantially the :spurious noises appearing ,withinth'e frequency response band of the Qearphone, and'fclarity, of ,reproduction in vthe hearing aid v'Whole is substantially increasefdf l, microphone rfspnse' at high' fquerciesshould ,have alare@ damping effect at' thcse'high frequencies butlow damping at low ,frequenciesfwhicli dictates fthe us off'a'y Aclothwhichis tightly woven, preferably from some s'olftf thread( The impedance 'of such cloth is" purely Vresistive and fthfe damping l effect increases directly with'the frecviiwiericyV of the sound applied to thecloth'sothat` little or nofattenuaf tionoccurs at the l'owffrequencies' Withinfthe reiwrlse, iresif'n' 0f 'theferphba whereas the/til desiredinois'es in" th'e`f`region'off4000 cycle'sfar'e unit as a YThe cloth lchcsenfor damping the severely attenuated'. y i a A vIt is within the'scope of 'thef presentinvention to use dampingmaterials othe'rthancloth woven from animar-or vegetable :obres as long as that ing from the" invention in itsbroader aspects, and, therefore',` the aim in theappendedclaim is to cover all such changes and modifications as fall #within the true spirit and scope'of this invention.

kI claim; VA hearing aid system comprising: a microphone for producing audio-frequency signals in response -toI impinging soundwaves and having a peak response ata predetermined natural mechanical resonancefrequency; an amplifier coupled to said microphonefor amplifying said signals; a sound reproducer coupled to said amplifier to convert said amplified signals into intelligible sound and having a response,rangeqsubstantially, excluding said :predetermined frequency, wherebycomponents of said wavesatesaidi predetermined-frequency tend'to overloadl said amplifier and Vproduce- Across-modulation with components` of said wavesl of frequencies g within; said,reproducerr re,- sponserange; and ank acoustic damper of;tightly Woven lsof-t cloth'-fibersgsupporteds in ,thepath of said soundwwaves` for, reducing said' peakfresponse to minimize said cros'semodulation and themesultantdistortion of said reproduced sound'.A 4

f "WALTERVA' SCHWALMQ,

REFERENCES CITEDv lrThe following references are of'recor'd Yin-"the file ofthispatent: v ,p `UNITED sTArEsfpArENTs y, Datef r 

